1. Field of the Invention
This invention relates generally to Analog to Digital (A/D) converters and, more specifically, to a driver circuit for low voltage operation of a Successive Approximation Register (SAR) based Analog to Digital (A/D) converter and a method therefor.
2. Description of the Prior Art
Currently in a Successive Approximation Register (SAR) based A/D converter, an SAR is used to signal a driver circuit to activate different columns of a capacitor array. The driver circuit is presently made up of a plurality of cells wherein each cell is used to drive a respective column of the capacitor array. In its simplest form, each cell houses an inverter which drives its respective column to either V.sub.H or V.sub.L.
The problem with using an inverter is that the lowest voltage the driver circuit may operate at is the lowest operating voltage of the inverter. The lowest operating voltage of an inverter is generally 2V.sub.T where V.sub.T is the threshold voltage of the transistors that comprise the inverter. Even at this level, one experiences slow response times.
The A/D converter generally has a digital voltage V.sub.dd and a digital ground. In general, the digital voltage V.sub.dd is the operating voltage of the integrated circuit (IC). For analog sampling there is generally a V.sub.H and V.sub.L on which the samples are based. Since the operating voltage of the inverter is 2V.sub.T, the inverters limit the sampling range V.sub.H to V.sub.L.
Therefore, a need existed to provide an improved driver circuit for an SAR based an A/D converter. The improved driver circuit must be able to operate at lower voltages than a standard inverter circuit. The improved driver circuit must also allow for a wider analog sampling range.